It is presently discussed that the visual quality of eyes having an implanted intraocular lens (IOL) is comparable with normal eyes in a population of the same age. Consequently, a 70 year old cataract patient can only expect to obtain the visual quality of a non-cataracteous person of the same age after surgical implantation of an intraocular lens, although such lenses objectively have been regarded as optically superior to the natural crystalline lens. This result is likely to be explained by the fact that present IOLs are not adapted to compensate for defects of the optical system of the human eye, namely optical aberrations. Age-related defects of the eye have recently been investigated and it is found that contrast sensitivity significantly declines in subjects older than 50 years. These results seem to comply with the above-mentioned discussion, since the contrast sensitivity measurements indicate that individuals having undergone cataract surgery with lens implantation lens will not obtain a better contrast sensitivity than non-cataracteous persons of an average age of about 60 to 70 years.
Even if intraocular lenses aimed at substituting the defective cataract lens and other ophthalmic lenses, such as conventional contact lenses, have been developed with excellent optical quality as isolated elements, it is obvious that they fail to correct for a number of aberration phenomena of the eye including age-related aberration defects.
U.S. Pat. No. 5,777,719 (Williams et al.) discloses a method and an apparatus for accurately measuring higher order aberrations of the eye as an optical system with wavefront analysis. By using a Hartmann-Shack wavefront sensor, it is possible to measure higher order aberrations of the eye and use such data to find compensation for these aberrations and thereby obtain sufficient information for the design of an optical lens, which can provide a highly improved optical performance. The Hartmann-Shack sensor provides means for analyzing light reflected from a point on the retina of the eye of a subject. The wavefront in the plane of the pupil is recreated in the plane of the lenslet array of the Hartmann-Shack sensor. Each lenslet in the array is used to form an aerial image of the retinal point source on a CCD camera located at the focal plane of the array. The wave aberration of the eye, in the form resulting from a point source produced on the retina by a laser beam, displaces each spot by an amount proportional to the local slope of the wavefront at each of the lenslets. The output from the CCD camera is sent to a computer, which then performs calculations to fit slope data to the first derivatives of 66 Zernike polynomials. From these calculations, coefficients for weighting the Zernike polynomials are obtained. The sum of the weighted Zernike polynomials represents a reconstructed wavefront distorted by the aberrations of the eye as an optical system. The individual Zernike polynomial terms will then represent different modes of aberration.
U.S. Pat. No. 5,050,981 (Roffman) discloses another method for designing a lens by calculating modulation transfer functions from tracing a large number of rays through the lens-eye system and evaluating the distribution density of the rays in the image position. This is repeatedly performed by varying at least one lens surface until a lens is found which results in a sharp focus and a maximum modulation transfer function.
U.S. Pat. No. 6,224,211 (Gordon) describes a method of improving the visual acuity of the human eye by successively fitting aspheric lenses to the cornea and thereby finding a lens that can reduce spherical aberration of the whole individual eye.
The methods referred to above for designing are suitable for the design of contact lenses or other correction lenses for the phakic eye which can be perfected to compensate for the aberration of the whole eye system. However, to provide improved intraocular lenses aimed to replace the natural crystalline lens, it would be necessary to consider the aberrations of the individual parts of the eye.
U.S. Pat. No. 6,050,687 (Bille et al) refers to a method wherein the refractive properties of the eye are measured and wherein consideration is taken to the contribution of the individual surfaces of the eye to the total wavefront aberrations. The method described herein particularly aims at analyzing the topography of the posterior corneal surface in order to improve refractive correction techniques.
There has recently been a focus on studying the aberrations of the eye, including a number of studies of the development of these aberrations as a function of age. In two particular studies, the development of the components of the eye were examined separately, loading to the conclusion that the optical aberrations of the individual components of younger eyes cancel each other out, see Optical Letters, 1998, Vol. 23(21), pp. 1713-1715 and IOVS, 2000, Vol. 41(4), 545. The article of S. Patel et at in Refractive & Conical Surgery, 1993, Vol. 9, pages 173-181 discloses the asphericity of to posterior corneal surfaces. It is suggested that the corneal data can be used together with other ocular parameters to predict the power and the asphericity of an intraocular lens with the purpose of maximizing the optical performances of the future pseudophakic eye. Furthermore, it was also recently observed by Antonio Guirao and Pablo Artal in IOVS, 1999, Vol, 40(4), 5535 that the shape of the cornea changes with age and becomes more spherical. These studies indicate that the cornea in the subjects provides a positive spherical aberration, which increases slightly with the age. On the other hand, the rotationally symmetric aberration of the anterior corneal surface does not seem to be different between younger and older eye according to results found by T Oshlka et al in Investigative Ophthalmology and Visual Science, 1999, Vol. 40, pp. 1351-1355. In Vision Research, 1998, 38(2), pp. 209-229, A Glasser et al. Investigated the spherical aberration of natural crystalline lenses from eyes obtained from an eye bank after the cornea has been removed. According to the laser scanner optical method used herein it was found that the spherical aberration from an older lens (66 years) shows positive spherical aberration, whereas a 10-year-old lens shows negative spherical aberration. In addition, Vision Research, 2001, 41, pp. 235-243 (G Smith et al) discloses that the natural crystalline lens appears to have negative spherical aberration when in the relaxed state. Smith et at suggest that because older eyes have a larger aberration, it is likely that the spherical aberration of the crystalline lens becomes less negative with age.
In Ophthal, Physiol. Opt., 1991, Vol. 11, pp. 137-143 (D A Atchison) it is discussed how to reduce spherical aberrations in intraocular lenses by aspherizing the lens surface. The methods outlined by Atchison are based on geometric transferring calculations, which do not consider diffraction effects and any variations in refractive index along the ray path in inhomogeneous elements. These calculations will lead to errors close to the diffraction limit. Also in WO 98/31299 (Technomed) a ray tracing method is outlined according to which the refraction of the cornea is attempted to be considered for the design of an intraocular lens. In view of the foregoing, it is apparent that there is a need for ophthalmic lenses that are better adapted or compensated to the aberrations of the individual surfaces of the eye and are capable of better correcting aberrations other than defocus and astigmatism, as provided by conventional ophthalmic lenses.